Metallic interconnect wiring conductors in semiconductor integrated circuit (IC) chips are such that as wiring pattern layout pitches shrink, metals increase in electrical resistance and interlayer dielectric (ILD) films increase in capacitance. These pose a serious problem as to on-chip signal transmission delays. In order to avoid this problem, it becomes inevitable to employ high conductivity material, such as copper (Cu), for wires while using a certain insulative material with low dielectric constants for dielectric films. The latter is known as low-dielectric constant insulative film, also called “low-K” insulator film. A typical approach to forming the metal wires is to utilize a damascene method having the steps of defining grooves or holes in a low-K insulator film, burying Cu therein by plating techniques, and then performing planarization. A well-known representative one of the low-K insulator film is a porous low-K dielectric film as formed by introduction of vacant holes or voids thereinto.
Prior known low-K insulator film processing methodology includes a method for performing film treatment by using a patterned resist layer as a mask, and a method of doing the same by use of a hard mask. In the case of the resist mask using method, a resist pattern with less edge rounding risks is obtainable. However, this advantage does not come without accompanying a penalty: the low-K insulator film can be damaged during a subsequent stripping or “ashing” process.
On the contrary, the hard mask-used patterning method is expected to be a promising approach in view of the fact that it requires no such ashing process and thus the film damageability is avoidable. Usually in this case, the etching of a hard mask is performed with a resist pattern as a mask, followed by execution of the etching of its underlying low-K insulator film after removal of the resist. During the ashing of this resist after having etched the hard mask, an oxygen (O2) gas is typically used (see JP-A-2002-203852). In the prior art ashing process using the O2 gas at high temperatures, the hard mask film would often be thermally damaged, resulting in the hard mask becoming a SiO2-like film. In the next step of etching the low-K insulator film, the selection ratio required for the hard mask can decrease undesirably.
Another known approach is to use an ammonia (NH3) gas or a mixture gas of nitrogen (N2) and hydrogen (H2) to perform the ashing at temperatures lower than or equal to 25° C. under a pressure of 0.1 Torr or below (see JP-A-2002-043422). However, this approach with such the ashing conditions is faced with a problem which follows: a previously processed or “pretreated” hard mask can experience unwanted edge rounding—this is sometimes called “shoulder sloping”—by the influence of residual ions in a plasma, by way of example. The hard mask's rounded edge shape becomes more noticeable in the next low-K insulator film processing step.